Modern communications techniques transmit information efficiently by modulating the information before transmission. Modulation results in signal segments (generally referred to as symbols) which may represent a single bit or multiple bits of information. For example, in QPSK modulation, each symbol represents two bits of information. The symbols are grouped into sets containing predetermined numbers of symbols. The sets are called frames and each frame typically contains specific header information including, for example, identification, routing, or error detection/correction coding. Furthermore, the symbols are typically frequency shifted by a carrier into an appropriate band (for example, the bandwidth allocated to an FM radio channel) before or during transmission.
At the receiver, the transmitted signal must be acquired before the information may be extracted. Acquiring a signal includes determining the carrier frequency, determining the symbol timing, and determining the frame timing so that the receiver may synchronize with the transmitted signal. Once the transmitted signal is acquired, the receiver must maintain synchronization as well. Acquiring the transmitted signal and maintaining synchronization, however, are often extremely difficult.
In determining the carrier frequency the receiver must be able to account for many types of masking and interference. For example, where the receiver or transmitter is moving relative to the other, the true carrier frequency may be masked by a Doppler shift. Other factors may also affect the carrier frequency, including inaccurate time bases at the transmitter, atmospheric conditions such as ambient temperature, and multi-path interference.
Assuming that the receiver has acquired the carrier frequency, the receiver must then synchronize with the symbol timing in the transmitted frame, a process often referred to as clock recovery. In the past, clock recovery has typically involved trial and error demodulation of the transmitted signal at the receiver in order to determine where individual symbol modulation begins and ends. For example, when a particular trial demodulation yields incorrect data, the receiver either advances or retards its approximation to the symbol timing and makes another attempt. Once the receiver has acquired the symbol timing, it next has to determine the frame timing.
Acquiring frame timing is analogous to determining where a complete message starts and stops. Knowing where the message starts allows the receiver to examine the frame header information commonly included with the frame. The frame header information, for example, is often important in determining what, if anything, the receiver should do with the frame. In general, once the symbol timing is acquired, the receiver may monitor the transmitted data until it recognizes the next start of frame.
Because a receiver typically does not acquire the carrier frequency and symbol timing immediately, numerous symbols or frames may pass by before the receiver is able to recover information. In burst communications, in which data is transmitted in short segments or bursts, the delay incurred at the receiver to acquire the carrier frequency, symbol timing, and frame timing may prevent the receiver from recovering any data at all. Similarly, data may also be lost in continuous transmission systems during receiver start up.
In both continuous and burst transmission systems, the receiver may also lose data trying to resynchronize to the transmitted data. Resynchronization is necessary, for example, when a drop out occurs during reception. Typical sources of drop out include physical obstructions in the signal path, for example trees, buildings, and tunnels, as well as atmospheric and electrical disturbances between the receiver and transmitter. In order to cope with drop outs, transmitters typically intersperse additional synchronization information during the transmission of normal data.
In the past, during synchronization and resynchronization, receivers have typically employed a frequency sweep technique in order to acquire the carrier frequency. In the frequency sweep technique, the receiver hypothesizes the correct carrier frequency and searches many frequencies over a predetermined uncertainty range. At each hypothesis, the receiver must try to acquire symbol timing and frame timing. If the hypothesis fails, the receiver must continue trying to acquire the carrier frequency. In the past, therefore, the frequency acquisition process often requires substantial time and processing power.
To help receivers acquire the carrier frequency, transmitters typically transmit long preambles or headers of modulated information before the frame. The headers required to allow receivers to acquire the carrier frequency with acceptable probability often introduce an overhead of as much as 30% compared to the actual data in a frame. Thus, a significant amount of bandwidth and processing time is used simply to allow the receiver to acquire the transmitted signal as opposed to actually communicating useful information. In fact, the processing power required to acquire and maintain synchronization with the transmitted signal may surpass that required to decode the actual data by a factor of 10 or more.
Therefore, a need remains in the industry for an improved signal acquisition method which overcomes the disadvantages discussed above and previously experienced.
It is an object of the present invention to allow a receiver to acquire a transmitted signal.
It is a further object of the present invention to provide a receiver with an auto-correlation technique that may be used to acquire a transmitted signal.
It is another object of the present invention to reduce the time required for receiver to acquire a transmitted signal.
It is another object of the present invention to reduce the processing required for receiver to acquire a transmitted signal.
Still another object of the present invention is to reduce the cost and complexity associated with transmitters and receivers in a communications system.
It is an object of the present invention to significantly reduce the header required for a receiver to accurately acquire a transmitted signal.
It is yet another object of the present invention to allow a receiver to determine the carrier frequency and frame timing of a transmitted signal using symmetric chirp signals.
The signal acquisition technique of the present invention includes transmitting an auto-correlating header (xe2x80x9cheaderxe2x80x9d) followed by a data block of predetermined length followed by a symmetric auto-correlating (xe2x80x9ctrailerxe2x80x9d). A series of such framed data blocks comprises a data frame. The lengths of the header and trailer are predetermined and may, for example, be implemented as symmetric chirp (swept frequency) signals.
At the receiver, a header reference segment (xe2x80x9cheader referencexe2x80x9d) and a trailer reference segment (xe2x80x9ctrailer referencexe2x80x9d) of the auto-correlating header and auto-correlating trailer are stored. Upon reception of the transmitted header, data block, and trailer, the receiver correlates the header reference with the header thereby providing a header correlation signal. In addition, the receiver correlates the trailer segment with the trailer to provide a trailer correlation signal. Peaks in the header correlation signal and trailer correlation signal are examined in conjunction with the known predetermined data block, header, and trailer lengths to acquire the data blocks and data frame. In particular, synchronization is determined for the individual data blocks comprising frames. In addition, the carrier frequency offset of the data blocks and data frame as well as the timing and positioning of the data frame itself are determined.